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I am trying to get 3 large posters printed as decorations for an ongoing promo project, but as I attempt to upload the photos using this printing site tool, I am being told that the DPI (??) of the images is not sufficient (min requirement being 300). Maybe someone knows how do I change the DPI of the images? Can I do that with the shots attacheds, or is that not possible?

If you are going to print an 800x600 image at 300 dpi (dots per inch) that means the final picture will be 2.66"x2" - not exactly poster-sized.

You say your original images are twice as big, so if they are 1600x1200 pixels, then a 300 dpi print will be 5.33"x4" - roughly the size of a standard photoprint. Of course you can print the images bigger with fewer pixels per inch, but the quality of the image will get progressively worse the more you enlarge it. Speaking as someone in his 40s who wears glasses, I don't notice a big difference in quality going from 300 to 200 dpi unless I look carefully, but if you go much below say 150 dpi the picture will start looking "pixelly" and blurred. We are much more aware (or perhaps less tolerant) of this effect in printed images than we are on computer monitors, where "standard resolution" seems to be 96 dpi (ppi really).

If you want to print high quality images at large sizes, you probably need more pixels in the original image. Having said that, there is more to human perception than just maths, and you may well find that you can still get pleasing results for your application, so just upload you pictures, ignore the warnings and print one off to see how it comes out. You may find this link http://www.kenrockwell.com/tech/mpmyth.htm interesting, I don't buy everything he says but some of it makes sense.

It's pretty difficult to increase the pixel size of a photograph without reducing the quality of the image, because most image processing software will create new pixels based on interpolation (guessing), which can result in jagged lines and fuzzy edges.

Your best bet is to start with an image that came straight out of the camera at the same pixel size the camera used. For example, if your camera shoots images that are 1,024 pixels wide, make sure the image you send to the printer is 1,024 pixels wide. But if the printer requires 300 dots per inch, the widest he'll be able to print that image (without loss of quality) is 3.4 inches wide (1,024 / 300). If you need a poster that's (for example) 24 inches wide, you'll need an image file that's 7,200 pixels wide (24 * 300).

It's not likely that you have a camera that produces 7,200-pixel-wide images, so you have no choice but to use interpolation. Start with the widest image the camera produces and then use image editing software (like Photoshop or its competitors) to re-size the image to the required number of pixels, based on the width of your printed poster. You'll probably lose some quality in the process, but at least the print shop's website will accept the image.

I really appreciate all the input about this issue. The resized images here are half of what I had in my email. Luckily the person who took the shots still has the original size copies on their memory card. I will check the size of those photographs for the optimal solution.

MartinG and SkyDancer - thank you for such insightful replies, I've gotten a crash course in the subject, I really appreciate your help.

Okay, I am kind of awake now (heh). I agree on a lot of what's already been said, but if you have the right-out-of-the-camera file and if it was shot with enough pixels, then you should be ok. These large format printers have RIP software which generally deals very well with interpolation issues. Amazingly well, actually.

If you can send me the original file via email (in my profile), I'd be happy to take a look at it.

It's all a matter of how the prints will be viewed. If they are close to the viewer, then they'll see pixels (or grain, if it's a film image). The more distance there is between the print and the viewer, the less likely the average human eye is to see such detail.

Hold an average 4"x6" print at normal viewing viewing distance. Maybe 9"-12" from your face? Now look past the print at the wall across the room. Visualize the same image, the same size in your field of vision, but stuck up on the wall. How big a print would it be? 20"x30"? Bigger? The source image resolution of the 4x6 and the 20x30 could be exactly the same. You'd only see the difference when you walk right up to the 20x30. (or look at the 4x6 with a magnifier).

If the big prints need to be viewed up close, and look sharp, then they need to be made from a high resolution original image. If they will be viewed at a distance, no big deal.

For photographic printing 200ppi is usually the acceptable lower limit for prints viewed up close. For subjects with a lot of detail, 250ppi or more is better. 300ppi is usually about the maximum useable data for most photo printing processes.

I guess part of the issue, which wasn't completely clear in the original post, is whether the website he's dealing with is rejecting the file as too small or merely warning him about it.

If the latter, my advice would be to send the original file from the camera, ignore the warning and see how it turns out. As Allen writes, from a distance (which is how posters are usually viewed) it might be just fine.

If, OTOH, the print shop's website won't even accept a file that doesn't have 300 pixels per every inch of the final, printed product, then he'll have to use software to re-size the image and create a file that the website will accept.

Not much on the website your links leads to -- looks like a full service print shop and looks like a minimum of 100 prints might have to be ordered. Price seems okay. Maybe another company dealing with short-run jobs is a better way to go. I was looking around for something in your area-- found one, but I don't know anything about this company. They do look good, though, and they're in London. This is a link to their really good file prep guide-- it will help with what you need to do. With their equipment, you would likely be able to get away with a file at 150ppi.: http://www.londonprint.co.uk/assets/Uploads/pdf/file-supply-guide.pdf

If you want to stick with the company you linked in the original post, I've got some questions:- What size do you want to print?- How big a run (how many are you printing?)- Have you already contacted the company to get their uploaded file requirements?- Do you need to allow for full bleed (cropmark stuff) or will they deal with it without charging extra?- Is it four-color (files need to be converted to CMYK) ?- Large format inkjet or traditional press?- Only PDF files?- Any text needed for your poster?

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